Make Skybox not break rendering if its image is default or 2D.

[kpreid]

Objective

• Partially addresses Skybox::default() causes an internal error #23688.
• Gives advice to users who accidentally create an invalid Skybox.
• Prevents rendering from totally breaking in that case.

Solution

If the image is detected to be invalid, have prepare_skybox_bind_groups ignore it (and warn once).

Testing

Manually tested that examples/3d/skybox.rs works if the initial skybox has the default image.

If there’s a good way to programmatically test things like this, let me know — I didn’t see any similar tests to emulate.